1. Field of the Invention
The present invention relates, in general, to low voltage integrated circuits, and, more particularly, to systems and methods and circuits for implementing an on-chip temperature sensor in low voltage integrated circuits and semiconductor devices.
2. Relevant Background
Thermal performance is becoming an increasingly important characteristic of semiconductor devices such as integrated circuits (ICs). Solid state devices behave differently at different temperatures. Hence, the effects of temperature on integrated circuits and systems can significantly affect the operational characteristics of these circuits and systems. For example, when the on-chip temperature changes the electrical characteristics of the solid state devices change significantly, such as threshold voltage, wiring/contact resistance, electron mobility, and the like. ICs are typically designed to operate in worst-case temperature extremes. Such designs often sacrifice performance at normal operating conditions in order to ensure functionality under the extreme conditions.
A number of circuits and/or functional units in today's electronic devices are temperature sensitive and require accurate and reliable temperature information in order to compensate for temperature variations. For example, the system frequency may be reduced when a certain temperature threshold is reached in order to cause the temperature to be reduced below the critical point. Further, systems, such as portable electronic devices (games, laptops, notebook computers, personal digital assistants), and the like are sensitive to power consumption and may need to shut down all or part of their operations when the power, which is function of temperature, reaches a certain level. Additionally, some individual circuits may need to be disconnected or shut down when the temperature reaches a predetermined level. Self-protection mechanisms, for example, can engage to reduce power consumption and thereby keep the device within safe operating parameters. In other devices it may be desirable to increase or decrease operating frequency to compensate for changes in operating temperature. Also, internally generated voltages used by various subsystems in an IC are sensitive to temperature changes such that performance of those systems can be compromised unless there is some mechanism for compensating for the temperature variation. Accordingly, operating temperature of a semiconductor device such as an integrated circuit (IC) can be measured and used to control operation of the device according to the operating temperature.
As a specific example, the refresh period of a dynamic random access memory (DRAM) device is determined by the leakage current of the memory cells. The leakage current tends to increase as operating temperature increases. Hence, a warm memory device should be refreshed more frequently than a cool memory device. Conversely, the refresh frequency of a cool device can be reduced to save power without compromising data storage integrity.
As the feature sizes of integrated circuits are reduced, the maximum supply voltage these circuits can handle also goes down. While an older 0.7 μm CMOS process could operate at around 5V, a circuit fabricated in 0.18 μm CMOS, for instance, has a typical supply voltage of 1.8V or lower. These lower operating voltage makes the design of analog components, such as a temperature sensor components, more challenging. At lower operating voltages the characteristics of semiconductor devices that are used to fabricate temperature sensing circuits may become more inconsistent. While techniques are known to compensate for this increased variability in digital circuits, temperature sensing relies on analog properties of the semiconductor devices. Accordingly, fabricating stable temperature sensing circuits using low voltages remains problematic.
Hence, a need exists for a temperature sensor, methods for sensing temperature, and systems that incorporate such sensors and implement such methods that provides greater consistency and stability in low-voltage circuits.